The natural history and pathophysiology of microvascular complications in patients with insulin-dependent diabetes mellitus with onset of clinical disease in childhood will be investigated in a long-term longitudinal study. Metabolic and other characteristics of diabetics who develop microvascular disease as compared to those diabetics who do not develop these complications or acquire them at a significantly slower rate will be defined. A number of factors which may reflect the degree of diabetes-associated metabolic perturbations will be serially assessed. The development of clinical diabetic microvascular diseases will be carefully monitored by a variety of studies including fluorescein angiography, nerve electrophysiological studies and a series of renal function studies. The relationships between muscle capillary basement membrane width and both metabolic control and clinical microvascular disease will be investigated.